Various projecting devices, such as liquid crystal projectors, including image displaying elements, such as liquid crystal display elements, have been proposed. For example, Japanese Patent Laid-Open No. 2001-154152 discusses a projecting device including a reflective liquid crystal display element (liquid crystal on silicon (LCOS)).
In such a projecting device, a color separating-combining optical system is disposed between an illumination optical system and a projection optical system to project a color image based on lights of three colors, i.e., R-light, G-light, and B-light. A plurality of polarization beam splitters (PBSs) are arranged in the color separating-combining optical system together with dichroic mirrors or a dichroic prism.
The dichroic mirrors or the dichroic prism performs a color separating-combining operation, and light is separated or combined by the PBSs in accordance with the polarization state. Therefore, as described in Japanese Patent Laid-Open No. 2001-154152 and Displays, Vol. 23, 139, 2002, an optical system is constituted by a plurality of PBSs and dichroic prism/mirrors. In general, the color separating-combining function is provided by multilayered dielectric films or colorant.
Wavelength-selective optical filters using metal structures instead of the multilayered dielectric films or colorant are disclosed in U.S. Pat. No. 5,973,316 and Nature, Vol. 424, 14 Aug. 2003. These optical filters are hole-type optical filters in which openings are periodically arranged in a thin metal film and wavelength selection is performed by using surface plasmon. According to Nature, Vol. 424, 14 Aug. 2003, an RGB transmission spectrum can be obtained by a hole-type optical filter using the surface plasmon. More specifically, it is disclosed that a transmission spectrum with wavelengths of 436 nm (blue), 538 nm (green), and 627 nm (red) can be obtained by using a thin metal film with a sub-wavelength array of openings.
A color separating-combining optical system included in a projecting device, such as a liquid crystal projector described in Japanese Patent Laid-Open No. 2001-154152, has a problem that a large number of components are required. This is because the incident light is divided into s-polarized light and p-polarized light, which are output as outgoing light, in a specific wavelength range for which each PBS is designed, and both the s-polarized light and p-polarized light are reflected or transmitted in wavelength ranges other than the specific wavelength range. More specifically, as shown in FIG. 15, the p-polarized light and the s-polarized light are adequately separated from each other only in a wavelength range of, for example, 500 nm to 600 nm. In this wavelength range, the p-polarized light is transmitted while the s-polarized light is reflected. Therefore, a wavelength-selective filter or the like is necessary to use light in this wavelength range.
In other words, in the color separating-combining optical system, dichroic mirrors or a dichroic prism having a wavelength-selective transmitting function must be provided in addition to the PBSs to perform the light separating-combining operation for desired wavelength range. As a result, the number of components increases.
Therefore, it is desirable to provide an optical element having both the wavelength selecting function and the light separating-combining function in accordance with the polarization state.
In U.S. Pat. No. 5,973,316 and Nature Vol. 424, 14 Aug. 2003, holes are periodically arranged in a thin metal film having a relatively large area to provide a filter having a transmission spectrum that depends on the wavelength of surface plasmon induced on the metal surface.
However, in such a hole-type thin-metal-film filter, large light absorption occurs since the area occupied by the metal is large. Therefore, in the thin-metal-film filter described in U.S. Pat. No. 5,973,316, the transmittance is about 5 to 6 percent even at the highest peak.
To use the transmission spectrum obtained by such a filter with a relatively low transmittance, the intensity of the incident light must be increased to ensure the intensity of the transmission spectrum. Therefore, there is a possibility that the energy efficiency of a device including the hole-type filter will be low. In particular, although the amount of light absorption by the metal is relatively small in a microwave range, the amount of light absorption by the metal is large in a visible light range. Therefore, in the case where the hole-type thin-metal-film filter is used as a transmission filter for the visible light range, the scope of application of the filter to the actual device is limited.
Similarly, also when the hole-type filter is used as a reflection filter for the visible light range, the contrast of the reflected light relative to the transmitted light is low. Therefore, the hole-type optical elements including metal structure layers described in U.S. Pat. No. 5,973,316 and Nature, Vol. 424, 14 Aug. 2003 cannot be used as optical elements which provide sufficient amount of transmitted light or sufficient contrast of reflected light.